I did find that the vest runs a bit small, but that was perfect for me. I am 5'1" and 90 lbs, and the S/M vest fits just right over a running jacket. Since my winter running jacket is dark-colored, I needed something bright to wear over it in the woods.

I bought this and it arrived quickly. Fits perfectly and doesn't move around while running. I was always hesitant on buying a vest as I thought it would move around and annoy me. This vest is awesome and I have been wearing it for my nightly runs since it arrived.

I am 5'8 and 125 pounds and had no issues with the fit on the small vest. It is snug but I wouldn't like it to not be fitted or it would annoy me while running. I wear it over my running jackets and it isn't too tight. It does have velcro so you can adjust the sizing. It is very reflective and makes you easy to be seen in low light conditions.